參數(shù)資料
型號(hào)： AD9877ABSZ
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 27/36頁(yè)
文件大?。?/td> 0K
描述： IC PROCESSOR FRONT END 100MQFP
標(biāo)準(zhǔn)包裝： 1
位數(shù)： 12
通道數(shù)： 3
功率（瓦特）： 1.17W
電壓 - 電源，模擬： 3.3V
電壓 - 電源，數(shù)字： 3.3V
封裝/外殼： 100-BQFP
供應(yīng)商設(shè)備封裝： 100-MQFP（14x20）
包裝： 托盤(pán)
AD9877
Rev. B | Page 33 of 36
PCB DESIGN CONSIDERATIONS
Although the AD9877 is a mixed-signal device, it should be
treated as an analog component. The on-chip digital circuitry is
specially designed to minimize the impact the digital switching
noise has on the operation of the analog circuits. The power,
grounding, and layout recommendations in this section will
help provide the best performance from the MxFE.
COMPONENT PLACEMENT
Chances for obtaining the best performance from the MxFE are
greatly increased if the three following guidelines of component
placement are followed.
Manage the path of return currents flowing into the ground
plane so that high frequency switching currents from the
digital circuits do not flow onto the ground plane under the
MxFE or analog circuits.
Keep noisy digital signal paths and sensitive receive signal
paths as short as possible.
Keep digital (noise generating) and analog (noise
susceptible) circuits as far away from each other as possible.
To best manage the return currents, pure digital circuits that
generate high switching currents should be closest to the power
supply entry. This keeps the highest frequency return current
paths short and prevents them from traveling over the sensitive
MxFE and analog portions of the ground plane. Also, these
circuits should be generously bypassed at each device, further
reducing the high frequency ground currents. The MxFE
should be placed adjacent to the digital circuits, such that the
ground return currents from the digital sections do not flow
into the ground plane under the MxFE. The analog circuits
should be placed furthest from the power supply.
The AD9877 has several pins that are used to decouple sensitive
internal nodes. These pins are REFIO, REFB8, REFT8, REFB12,
and REFT12. The decoupling capacitors connected to these
points should have low ESR and ESL. These capacitors should
be placed as close as possible to the MxFE and be connected
directly to the analog ground plane.
The resistor connected to the FSADJ pin and the RC network
connected to the PLLFILT pin should also be placed close to the
device and connected directly to the analog ground plane.
POWER PLANES AND DECOUPLING
The AD9877 evaluation board demonstrates a good power
supply distribution and decoupling strategy. The board has four
layers: two signal layers, one ground plane, and one power
plane. The power plane is split into a 3 VDD section used for
the 3 V analog supply pins of the AD9877 and a VANLG
section that supplies the higher voltage analog components on
the board.
That 3 VDD section typically has the highest frequency
currents on the power plane and should be kept the farthest
from the MxFE and analog sections of the board. The DVDD
portion of the plane brings the current used to power the digital
portion of the MxFE to the device. This should be treated
similarly to the 3 VDD power plane and be kept from going
underneath the MxFE or analog components. The MxFE
should sit above the AVDD portion of the power plane.
The AVDD and DVDD power planes can be fed from the same
low noise voltage source. They should be decoupled from each
other, however, to prevent the noise generated in the DVDD
portion of the MxFE from corrupting the AVDD supply. This
can be done by using ferrite beads between the voltage source
and DVDD and between the source and AVDD. Both DVDD
and AVDD should have a low ESR, bulk decoupling capacitor
on the MxFE side of the ferrite as well as low ESR, low ESL
decoupling capacitors on each supply pin (for example, the
AD9877 requires 17 power supply decoupling caps). The
decoupling capacitors should be placed as close as possible to
the MxFE supply pins. An example of the proper decoupling is
shown in the AD9877 evaluation board schematic.
GROUND PLANES
In general, if the component placing guidelines discussed in the
Component Placement section can be implemented, it is best to
have at least one continuous ground plane for the entire board.
All ground connections should be made as short as possible.
This results in the lowest impedance return paths and the
quietest ground connections.
If the components cannot be placed in a manner that keeps the
high frequency ground currents from traversing under the
MxFE and analog components, it may be necessary to put
current steering channels into the ground plane to route the
high frequency currents around these sensitive areas. These
current steering channels should be made only when and where
necessary.
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